(a) Field of the invention
The present invention pertains to a method of growing Group II-VI compound semiconductor crystals, and more particularly it relates to a method of performing solution growth of ZnSe crystals.
(b) Description of the prior art
Group II-VI compound semiconductor crystals such as ZnS, ZnSe and CdS in general have energy band gaps greater than those of Group IV semiconductor crystals and of Group III-V compound semiconductor crystals, and also they are such crystals that the transition of carriers is of the direct transition type, and therefore the Group II-VI compound semiconductor crystals are given much expectation as a material for constituting semiconductor devices having interesting characteristics which are not obtained by other semiconductor crystals. Especially, a ZnSe crystal has an energy band gap of 2.67 eV at room temperature, and accordingly expectation is placed on the development, in the future, of LEDs (light-emitting diodes) having emission spectra in the wavelength region (450-490 nm) of blue color.
This ZnSe crystal, like other conventional Group II-VI compound semiconductor crystals, has been difficult to attain its crystal growth, and also controlling of its conductivity type and impurity concentration has not been easy.
That is, a ZnSe compound semiconductor crystal (hereinafter to be abbreviated as ZnSe crystal) is obtained by relying on such growth method as is featured by a melt growth at a high temperature which is typically represented by Bridgman method. In such case, Se vacancies are noted to have been generated in a large number in the ZnSe crystal thus grown, owing to the high vapor pressure of Se atoms. Because these Se vacancies act as the donor of the carriers, the ZnSe crystal which is produced by relying on said prior growth method has usually been of the n type having a high resistivity. Moreover, there has not been established the technique to control the value of this resistivity, and the present status in the art is such that, in addition to the abovesaid inconveniences, no p type crystal can be obtained in practical state.
Accordingly, while the useful properties of the ZnSe crystal have been appreciated, the technology has not developed so far to provide a device in which a practical pn junction is formed.
Even if a technique for the formation of a pn junction is established based on the conventional crystal growth, i.e. the melt growth method, and an LED consisting of a diode having a pn junction is formed, it should be noted that the vacancies of Se which is a Group VI element become combined with the atoms of the impurity present to serve as the non-radiative centers or form deep energy levels, so that the light-emission efficiency of such an LED will become very low, or there cannot be obtained no good LED but just an LED wherein the radiation through deep levels is dominant. From the foregoing facts, it has been desired to obtain a semiconductor crystal having a high crystal perfection which will never form such deep levels as referred to above, and also to establish a technique for forming a pn junction based on such crystal. Solution growth methods using various kinds of solutions have been attempted so far, but none of them has attained a ZnSe crystal having a high crystal perfection.
The present inventor, standing on the viewpoint that what is most important in the growth process relying on the method of performing a solution growth of such Group II-VI compound semiconductor crystal such as ZnSe crystal to attain the above-mentioned purpose and desire, is to control the deviation of the crystal from stoichiometric composition to a minimum value, earlier has invented a solution growth method which utilizes the temperature difference method using, as a solvent, a melt of that element having a higher vapor pressure among the crystal-constituting elements, and which disposes a source crystal in the higher temperature region of the solvent, performing recrystallization of the crystal in the lower temperature region, and also using the vapor pressure controlling method which applies, from outside of said solvent, the vapor pressure of the element having the lower vapor pressure, and applied for patent in Japan under Patent Preliminary Publication No. Sho No. 57-183400 entitled "A method and an apparatus for growing a Group II-VI compound semiconductor crystal". Here, the said earlier invention will be briefed concretely with respect to a ZnSe crystal which is relevant to the present invention. As discussed above, the conventional melt growth method performs the growth at a high temperature. Besides, in such method, Se atoms have a higher vapor pressure over Zn atoms, causing a shortage of Se atoms, and thus there could have obtained only those crystals exhibiting a great deviation from stoichiometric composition. As such, in said earlier invention, Se is used as a solvent to compensate for the shortage of Se within the crystal to be grown to thereby minimize the deviation of the crystal composition from stoichiometry, and also the vapor pressure of Zn is controlled externally so as to be optimum, to thereby make it possible to obtain a semiconductor crystal having a high crystal perfection and a high impurity concentration of the p type.
While said earlier invented method has markedly enhanced the technique of controlling the crystallinity, conductivity type and carrier concentration, there has not yet been established optimum manufacturing conditions for obtaining good crystals, because of the fact that the earlier invention provides for a new technique.